1. Field of the Invention
This invention relates to superconducting microwave components. More specifically, this invention relates to high frequency parts for treating electromagnetic waves having short wavelengths, such as microwaves, millimeter waves or others, and especially to new constitutions of microwave components having the conductor layers formed of oxide superconducting materials.
2. Related Background Art
Although the electromagnetic waves having wavelengths from tens centimeters to some millimeters are called microwaves, millimeter waves or others are theoretically only a part of the electromagnetic wave spectrum, these electromagnetic waves are, in many cases, specially studied independently in the engineering field because special means and parts have been developed for treating these electromagnetic waves. The microwave line for guiding the electromagnetic waves in this band comprises a pair of conductor lines arranged through a dielectric and having one of the conductor lines grounded.
On the other hand, in 1986 (La,Ba).sub.2 CuO.sub.4 which exhibits superconductivity at 30 K was discovered by Bednorz, Mueller, et al. In the next year 1987 YBa.sub.2 Cu.sub.3 O.sub.4 having a critical superconducting temperature in the order of 90 K was discovered by Chu, et al. In 1988 Maeda, et al. discovered the so-called Bi-based composite oxide superconducting material which exhibits a critical superconducting temperature exceeding 100 K. Since these composite oxide superconductors can realize superconductivity by their being cooled by inexpensive liquid nitrogen, the possibility of practical applications of the superconducting technique has been suddenly noted.
Microwave components also enjoy the characteristic phenomena of superconductivity. That is, generally in a strip line the attenuation constant of a conductor due to a resistance is proportional to a square root of a frequency. The dielectric loss also increases with an increase of frequency. The dielectric loss in the recent strip lines is almost attributed mainly to a resistance of a conductor layer especially in the band equal to or lower than 10 GHz owing to the improvement of dielectric materials. Accordingly it improves the efficiency of the strip line to decrease the resistance of a conductor layer of the strip line. That is, by making a conductor line superconducting, the propagation loss is much reduced while the applicable frequency band is expanded toward the higher frequency side.
Microwave strip lines not only can be used as mere transmission lines, but also can be patterned suitably to be microwave components, such as inductors, filters, resonators, delay lines, directional couplers, etc. Accordingly the improvement of strip lines leads to the improvement of the characteristics of such microwave components.
Since the use of oxide superconducting materials as superconducting materials enables superconductivity to be realized by use of inexpensive liquid nitrogen, it is possible that microwave components of higher performance will prevail in more various fields.
But it is impossible to obtain microwave components which sufficiently take advantage of the characteristics of superconductors, by simply replacing the metal conductors of microwave components with oxide superconductors.
One reason for this is that further decrease of the dielectric loss is necessary. That is, in the conventional microwave lines using metal conductors the dielectric loss in comparison with the conductor loss of the metal conductor has been sufficiently decreased. In the case where superconductors are used as the conductor lines, the decrease of the dielectric loss is again brought up as a problem to be solved since the conductor loss can be minimized.
On the other hand, it is known that oxide superconductors can have good characteristics when the superconducting films are formed on specific substrates, as of MgO, SrTiO.sub.3, etc. But all the oxides of MgO, SrTiO.sub.3, etc. do not have good characteristics of dielectrics. But when oxide superconducting films are formed on substrates, as of sapphire, SiO.sub.2, etc., having very low dielectric losses, the superconductive characteristics of the superconducting films are deteriorated or lost. Thus it is substantially impossible to form oxide superconducting films which are to be conductor lines, directly on these dielectric substrates of low dielectric losses. In short, it is impossible to fabricate microwave components which exhibit effective characteristics simply by replacing the conductor portions of the conventional microwave components formed of metal conductors with oxide superconductors.